Ferroelectric and Piezoelectric Properties of Blends of Poly(Vinylidene Fluoride Trifluoroethylene) and a Graft Elastome
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the two components. The solution was then cooled to room temperature, cast on glass substrates, and placed in a vacuum chamber. After drying overnight under vacuum, tackfree films were obtained. In order to increase their crystallinity, and possibly their remanent polarization, the blend films were thermally annealed at 140 C for 10 hours. The thickness of the films was around 20JLm. The films were tested using X-ray diffraction (XRD) and differential scanning calorimetry (DSC) in order to assess their composition and degree of crystallinity. Polingand Measurements: Gold electrodes were sputtered on the opposing surfaces of the films to establish electrical contact. The films were poled using a triangular wave signal with a peak value of IOOMV/m at 30 mHz. The modulus, Ell, and the piezoelectric strain coefficient, d 3l, were measured using a modified Rheovibron. The capacitance of the blend films was measured using a HP Analyzer 4192A, and the dielectric constant, c, was calculated from the value of the capacitance. These measurements were performed as a function of the relative composition of the blends (wt.% copolymer content), temperature, and frequency. RESULTS AND DISCUSSION Figure 1 shows the crystallinity as a function of copolymer content in the blend. The calculated crystallinity of the blend system is found from Xtotal- fcopolymerXcopolyrmer + felastomerXelastomer
(1)
wheref is the fraction of the components and X is the crystallinity. Both the measured and calculated crystallinities increase with increasing copolymer content in the blend, however, the measured crystallinity is lower than the calculated one. This indicates that the presence of both components in the blend may reduce their crystallization as compared to each individual one.
60 -50
Calculated--
X 2ý40
"20
Experimental
10 0
20
40 60 80 Copolymer content, (wt.%)
100
Figure 1. The relationship between the copolymer content and the crystallinity in the blends To determinate the remanent polarization, the measurement of the polarization, P, versus the electric field, E, was carried out. Corrections were made to eliminate the effects of conductivity on the ferroelectric hysteresis loops [7]. The measured remanent polarization, Pr, as a function of the copolymer content in the blends is shown in Figure 2 and compared with the remanent polarization calculated using the following equation: 96
(2)
Prtotal) -fopolyni-r P,•(cpolyrnr) +filastomwr Pr(elstomr)
wheref is the relative fraction of the components, P,(cooly,>) is the remanent polarization in the pure copolymer, P is the remanent polarization in the elastomer, and Pr(tota) is the resulting remanent polarization of the blend film. As can be seen, both the measured and the calculated remanent polarization increase with increasing copolymer content in the blends. The value of the measured remanent polarization is very close to the calculated one. This is an indication of the linear relationship between Pr and the polar crystallinity in the blends. 60 50
M
40
0
CL
-
Calculat
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